Isomaltulose (or palatinose) is a reducing disaccharide having the systematic name of 6-O-.alpha.-D-glucopyranosyl-D-fructofuranose. Isomaltulose has been proposed for use as a sweetener in the food industry and it is a raw material for the production of isomalt (palatinit) by hydrogenation. Isomalt is a substantially equimolar mixture of .alpha.-D-glucopyranosyl-(1,6)-sorbitol and .alpha.-D-glucopyranosyl-(1,6)-mannitol. Isomalt is a non-cariogenic special sweetener described as having a gentle sweetening flavour.
Various processes for the isomerization of sucrose to isomaltulose have been reported. The isomerization is believed to be performed by .alpha.-glucosyl transferase (saccharose mutase) enzyme which has been found to exist in micro-organisms such as Protaminobacter rubrum, Serratica plymuthica, Erwinia rhapontici, etc. The known isomerization techniques include isomerization with viable or dead micro-organism cells or with the enzyme in extracted form. Various techniques for the immobilization of the enzyme have also been reported.
Thus, for instance EP-B1-0 028 900 suggests immobilization of the enzyme of various isomerizing micro-organisms, preferably E. rhapontici. The preferred immobilization technique comprises entrapping dead microbe cells within calcium alginate pellets. Immobilization in a thick aqueous slurry of diethylaminoethyl cellulose (DEAE-cellulose) is also disclosed but is reported to provide poorer results.
U.S. Pat. No. 4,386,158 discloses improving the physical strength of calcium alginate gels used to immobilize .alpha.-glucosyl transferase by treatment with polyethyleneimine and glutaraldehyde.
U.S. Pat. No. 4,640,894 describes the production of isomaltulose by using a reactor including immobilized dead cells of P. rubrum. Various immobilization techniques, such as entrapment in gels and flocculation with flocculants are disclosed.
U.S. Pat. No. 4,373,313 and EP-B-0 077 971 disclose the immobilization of viable cells of P. rubrum by flocculation with tannin and a long chain polyamine, reaction with an adduct of epihalohydrin/polyamine copolymer and glutaraldehyde, and drying.
EP-B-0 049 801 discloses the immobilization of a sucrose converting enzyme isolated from P. rubrum on various carrier materials such as hollow fibers or cation exchange resins.
EP-B-0 200 069 discloses the selective immobilization of extracted saccharose mutase enzyme on an anionisable carrier material, especially a sulphonic acid cation-exchange matrix.
EP-A-0 160 253 discloses immobilization of P. rubrum cells through entrapment in a polymer system during polymerization to provide biocatalysts for the conversion of sucrose to isomaltulose.
The prior art processes for the conversion of sucrose to isomaltulose are not entirely satisfactory. The carrier material is generally produced on-site. The immobilization processes are complicated and care must be exercised not to harm the sensitive enzyme. Entrapping in alginate or carrageenan gels requires several separate process steps to be performed and a final cross-linking with glutaraldehyde is often necessary. The prior art flocculation with a flocculating agent in solution requires several subsequent production steps involving the enzyme, including drying, extrusion and granulation. Inclusion of the enzyme in a polymer during polymerization may provide a physically strong product but this process is also complicated and may affect the activity of the sensitive enzyme.
A further disadvantage associated with the prior art methods of including viable or dead micro-organism cells in a carrier during the production of the carrier matrix itself lies in the fact that when the activity has been lost, the whole carrier must be discarded.